In known filter devices of this kind the fibre suspension to be separated is supplied to the container, such that the filter body is at least partly immersed in the suspension. The hydrostatic pressure in the fibre suspension in the container forces a fine fraction of the fibre suspension through the part of the wall of filter material of the filter body which is immersed in the fibre suspension, so that fibres are deposited and form a layer of fibres on the filter material. The formed layer of fibres constitutes in itself a filter medium, which is substantially tighter than the filter material of the filter body and which merely allows water from the fibre suspension in the container to pass through.
A filter device for thickening fibre suspensions is known from U.S. Pat. No. 4,138,338, which discloses a disc filter with an inlet tank for a fibre suspension to be thickened. From the inlet tank fibre suspension flows directly into a container, in which the discs are immersed in fibre suspension. To increase the dewatering capacity of the disc filter a number of spray members is arranged to spray liquid, for instance a quantity of the fibre suspension to be thickened, onto the disc walls of filter material which are immersed in the fibre suspension in the container, so that creation of tight fibre layers on the walls is prevented.
The known filter devices, as described above, are not suited for the separation of undesired relatively small particles from fibre suspensions, since most of such small particles would be trapped by the created tight layer of fibres deposited on the filter material or by thickened fibre suspension during the separation. Consequently, in practice such known filter devices usually have only been used for thickening of fibre suspensions, i.e. mere dewatering of the latter. Conventionally, undesired relatively small particles are therefore initially separated from the fibre suspensions by other kinds of separation devices, for instance by flotation plants. Subsequently, the fibre suspensions thus cleaned can be dewatered, for instance by known filter devices of the kind described above.
When producing paper from waste paper pulp the undesired relatively small particles are substantially composed of printing ink. They are separated from the waste paper pulp to avoid greyness of the produced paper. Hitherto, the profitability of producing paper from such a waste paper pulp has been poor. However, the authorities tend to tighten up the requirements on paper manufacturers to produce some paper from waste paper pulp, in order to provide a reduction of the paper waste and a saving of raw wood material.